1. Field of the Invention
The invention relates to a process for the production of rigid polyurethane foams from at least difunctional polyisocyanates, polyols with at least two hydroxyl groups per molecule whose equivalent weight per hydroxyl groups is about 50 to 800, catalysts, blowing agents and polysiloxane polyoxyalkylene block copolymers as foam stabilizers and, if necessary, conventional additives.
2. Description of the Prior Art
Rigid polyurethane foams are produced in large quantities by conventional processes using known raw materials. A summary of the raw materials which may be used and the applicable processes may be found in Ullmann's "Enzyklopadie der technischen Chemie" (Encyclopedia of Chemical Engineering), 1980, Vol. 19, pages 301 to 341, and Kunststoff-Handbuch (Plastics Handbook) Vol. VII, Polyurethanes, by R. Vieweg and A. Hochtlen, Carl Hanser Verlag, Munich, 1966, pages 504 to 544, as well as in "Polyurethanes" Kunststoff-Handbuch (Plastics Handbook) 7, by G. Oertel, Carl Hanser Verlag, Munich, 1983, pages 246 to 331.
Rigid polyurethane foams are preferably used for thermal insulation. They may also be foamed into molded articles, such as, panels. Additionally, it is possible to fill hollow spaces of all types with foam, in which case, the foam in the hollow space acts as a structural element and then must also have load-bearing properties. For example, filling doors and housings of refrigerators, or the hollow spaces of surfboards, or structural members, pipes, and automobile components with foam is known.
In the foaming process, the mixture of materials to be foamed is introduced into the hollow space which is to be filled with foam. The rising foam must then fill the hollow space of the mold uniformly and be in the form of fine cells before it cures. However, defects are frequently formed in the foam when such hollow spaces or molded articles are foamed. Such defects may occur in the form of small holes or voids. Frequently, destroyed cell structures or areas of different foam density are observed, especially in the edge regions. The bond of the foam to the surface of the inner wall of the mold is especially weakened by these defects, so that the mechanical properties of the foamed molded part as a whole are affected.
It was found that the quality and the properties of the rigid foam formed depend in a large degree on the structure and the chemical composition of the foam stabilizer used. For this reason, polysiloxane polyoxyalkylene block copolymers of different structure and different composition have already been used as foam stabilizers.
Polysiloxane-polyoxyalkylene block copolymers used for the aforementioned purpose include hydrolysis-resistant siloxane-oxyalkylene copolymers having the general formula: ##STR2## wherein l=3 to 25; x=1 to 25; y=0 to 15; z=2 or 3; p=1 to 10; R=hydrogen or methyl, with the proviso that at least 25 weight percent of the oxyalkylene groups are oxyethylene groups and, when R=H, the hydroxyl groups comprise at least 1.5 weight percent of the copolymer. These are described, for example, in German Auslegeschrift No. 17 19 238.
According to German Pat. No. 20 29 293, siloxane-modified carbamic acid derivatives are used for the production of polyurethane foams. These carbamic acid derivatives comprise at least one structural unit having the following general formula: ##STR3## to which other structural units having the general formula: EQU R.sub.2 'SiO.sub.2/2
are linked by Si-O-Si bridges with at least one of a thousand structural units corresponding to the first-mentioned formula and wherein
R' represents C.sub.1 -C.sub.10 -alkyl, C.sub.4 -C.sub.10 -cycloalkyl or C.sub.6 -C.sub.10 -aryl radical which, optionally, may be halogenated or cyano-substituted; PA0 R" represents a hydrogen atom, or a methyl or phenyl radical; PA0 R"' represents a C.sub.1 -C.sub.10 -alkyl, C.sub.4 C.sub.10 -cycloalkyl, C.sub.2 -C.sub.10 -alkenyl, C.sub.7 -C.sub.10 aralkyl, di-(C-C.sub.10 -alkyl)amino-C.sub.1 -C.sub.10 -alkyl, C.sub.6 -C.sub.10 -aryl or C.sub.7 -C.sub.10 alkaryl radical; PA0 Q represents a saturated C.sub.1 -C.sub.6 alkyl radical or C.sub.6 -aryl radical; PA0 m is 1 or 2; PA0 n is 2, 3 or 4; and PA0 b is a whole number from 1 to 200; PA0 B is a methyl radical or an A.sup.1 or A.sup.2 polyoxyalkylene block;
and wherein these radicals and numerical values are independent of each other at each side in a molecule.
Additional, particularly suitable polysiloxane-polyoxyalkylene block copolymers are disclosed in German Pat. No. 16 94 366. The patent describes a process for the production of polyurethane foams wherein polyoxyalkylene-polysiloxane block copolymers whose polysiloxane block is synthesized by known methods is used. However, the polyoxyalkylene block consists of 25 to 70 weight percent of a polyoxyalkylene with an average molecular weight of 1,600 to 4,000 and an ethylene oxide content of 20 to 100 weight percent, the remainder being propylene oxide and, optionally, higher alkylene oxides; and 30 to 75 weight percent of a polyoxyalkylene with an average molecular weight of 400 to 1,200 and an ethylene oxide content of 65 to 100 weight percent, the remainder being propylene oxide and, optionally, higher alkylene oxides. Therefore, this patent is based on the fact that polyoxyalkylene blocks of defined, different construction and therefore different hydrophilicities, are contained in the block copolymer.
Practice has shown that foam stabilizers, known from the state of the art, do not always produce satisfactory rigid polyurethane foams and that defects occur time and again in the edge zones, which defects affect the mechanical properties of the foam-filled molded articles and also lower the thermal insulation.